1. Field of the invention
This invention relates generally to a display circuit of an oscilloscope in which a cathode-ray tube (CRT) is employed.
An oscilloscope is used in order to observe various signal waveforms. On a surface of a display of the oscilloscope, not only waveforms but also characters, which show various data for observation, are simultaneously displayed. This invention relates more particularly to a display circuit to deflect a CRT of such an oscilloscope.
2. Description of the Prior Art
The prior art of a circuit to display signal waveforms to be observed and characters is shown in FIG. 1.
Numerals 11 and 12 indicate signal input terminals. Each signal input terminal of 11 and 12 is applied a signal of a polarity reverse to each other from a prestage amplifier which is not shown in FIG. 1.
Numeral 13 identifies a main amplifier which amplifies signals from input terminals 11 and 12. The amplified signals are amplified to bases of transistors 31 and 32 via transistors 16 and 17. Numerals 18 and 19 denote load resistors connected to a power supply V.sub.4 for the main amplifier 13 and transistors 16 and 17 which form the common-base circuit.
Base of the transistors 16 and 17 are connected together to biasing resistors 76 and 77 which divide a voltage of a power supply V.sub.3. Transistors 31 and 32 and resistors 33, 34 and 35 construct a driving amplifier 101 in the common-emitter connection. Transistors 36 and 37 in the common-base connection form a final stage amplifier 39 of a balanced type. The final stage amplifier 39 is driven by the driving amplifier 101.
The resistor 33 is connected between emitters of transistors 31 and 32. The resistors 34 and 35 are respectively connected between emitters of transistors 31, 32 and the ground. The signal applied between bases of transistors 31 and 32 is amplified to be added to deflection plates (not shown) of the CRT 38 through the final stage amplifier consists of transistors 36 and 37 so that signal waveforms are displayed on the CRT 38. Numerals 58 and 59 designate load resistors of the driving amplifier 101 and the final stage amplifier 39. The resistors 58 and 59 are connected to a power supply V6. Bases of transistors 36 and 37 are connected together to a power supply V.sub.5 in order to be applied a bias voltage.
Emitters of transistors 14 and 15 are connected between emitters of transistors 16 and 17 which are connected to outputs of the main amplifiers 13, in which collectors of transistors 14 and 15 are connected together to a power supply V.sub.1. Bases of transistors 14 and 15 connected together are applied a character switching signal.
Numerals 21 and 22 indicate character input terminals. Each terminal 21 and 22 is applied a character signal of a polarity reversed to each other from a character signal generator which is not shown in FIG. 1.
Numeral 23 identifies a character amplifier which amplifies the character signal from character input terminals 21 and 22. The character signal amplified by the character amplifier 23 is applied to bases of transistors 31 and 32 of the driving amplifier 101 through transistors 26 and 27 which form the common-base circuit. Load resistors of the character amplifier 23 are resistors 18 and 19 which are used in common as load resistors of the main amplifier 13.
The signal applied between bases of transistors 31 and 32 is amplified to be added deflection plates of the CRT 38 through transistors 36 and 37 so that characters are displayed on the CRT 38. Emitters of transistors 24 and 25 are respectively connected to emitters of transistors 26 and 27 which form the common-base circuit. Collectors of transistors 24 and 25 are connected together to the power supply V.sub.1. Bases of transistors 24 and 25 connected together are applied a bias voltage obtained by resistors 78 and 79 dividing the voltage of the power supply V.sub.3.
Bases of transistors 26 and 27 are connected together to bases of transistors 14 and 15 and an emitter of a transistor 50 which forms an emitter follower with resistor 49.
A switching signal is applied from a character switching signal generator 71 to the base of the transistor 50 forming the emitter follower. The character switching signal generator 71 generates a signal which is synchronized with a timing signal applied from a character signal generator not shown in FIG. 1. The switching signal applied to the base of the transistor 50 has two levels of "H" and "L".
Base voltages respectively applied to transistors 16, 17 and 24, 25 are set a voltage which is nearly center in levels of "H" and "L" for switching signal.
When the switching signal shows a level of "H", the base voltage of transistors 14 and 15 become higher than the base voltage of transistors 16 and 17, so that transistors 14 and 15 are turned on.
Transistors 16 and 17 are, therefore, turned off and the signal from the main amplifier 13 is not applied to bases of transistors 31 and 32. On the other hand, base voltage of transistors 26 and 27 becomes higher than base voltage of transistors 24 and 25 so that transistors 24 and 25 are turned off. Transistors 26 and 27 are, therefore, turned on to allow flow of their collector currents. The signal amplified by the character amplifier 23 is applied to bases of transistors 31 and 32. The signal is further amplified by the final stage amplifier 39 to display characters on the CRT 38.
When the output of the character switching signal generator 7 shows a level of "L", the base voltage of transistors 26 and 27 becomes lower than the base voltage of transistors 24 and 25.
Transistors 26 and 27 are turned off and transistors 24 and 25 are turned on, so that the signal from the character amplifier 23 is not applied bases of transistors 31 and 32. On the other hand, the base voltage of transistors 16 and 17 becomes higher than the base voltage of transistors 14 and 15 so that transistors 14 and 15 are turned off. Transistors 16 and 17 are, therefore, turned on to allow flow of their collector currents. The signal amplified by the main amplifier 13 is applied to bases of transistors 31 and 32. The signal is further amplified by the final stage amplifier 39 to display signal waveforms on the CRT 38.
As above-description, various characters and signal waveforms are switchingly displayed on the CRT 38 by the output from the character switching signal generator 71.
In the circuit of FIG. 1, the amplifier 101 employing the common-emitter connection and the final stage amplifier 39 are used in common in order to display signal waveforms and characters on the CRT 38.
In an amplifier of differential inputs employing the common-emitter connection, the location of circuit elements are symmetrical with respect to the input signal in order to eliminate influences of temperature variations and others. As voltages and currents of circuit elements with the balanced location vary in accordance with the level of the input signal, power dissipations of those elements vary so that those calorific values vary.
Temperature differences are, therefore, caused between those elements and operations of those elements having temperature coefficients are influenced by them. Accordingly, the signal amplified by those elements does not faithfully follow the input signal, but also a margin of error caused by their temperature coefficients is added on to the amplified signal.
This influence of the temperature coefficients causes a kind of drift on the CRT 38. For example, when two horizontal traces are displayed on the CRT 38, a trace of those is shifted up or down by turning a vertical positioning knob and the other trace is also drifted up or down in a little dimension although another vertical positioning knob is not turned. When signal waveforms and characters are simultaneously displayed on the CRT 38 by switching under control of the character switching signal generator 71, the same drift phenomena can be observed. The drift phenomena are caused by differences between levels of the signal waveforms and the signal of characters which are applied to the driving amplifier 101.
As characters need detailed display, when the drift phenomena cause fluctuations of characters on the CRT 38, characters are sometimes not readable.
In order to resolve such a problem, resistors have been appended between collectors of transistors 31, 32 and emitters of transistors 36, 37 respectively, to obtain a temperature balance in transistors 31, 32, 36 and 37.
Those appended resistors have, however, injured a dynamic range of amplifiers 101 and 39 in a high frequency range. Further, a resistor and a capacitor connected in series have been appended between emitters of transistors 31 and 32 to eliminate the drift phenomena by a time constant of the resistor and capacitor, however the sufficient elimination effects have not been obtained.